1. Field of the Invention
The present invention relates generally to nuclear reactor coolant system pumps and, more particularly, is concerned with an improved reactor coolant pump having an internal self-cooling arrangement.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and reactor coolant pumps.
In one version of the reactor coolant system used in a nuclear power plant, the reactor coolant pumps are high inertia pumps hermetically sealed and mounted to the one steam generator in the respective coolant loop. Each pump has an outer casing, a central axially extending rotor rotatably mounted at its opposite ends by upper and lower bearings, and a canned motor located about the pump rotor between the upper and lower bearings. The motor includes a rotor section mounted for rotation on the pump rotor and a stator stationarily mounted to the casing about the rotor section. An impeller mounted at one end of the pump rotor rotates therewith and draws reactor coolant water axially through a central inlet nozzle in the pump casing and discharges the water tangentially through an outlet nozzle in the pump casing.
The temperature of the reactor coolant water is typically in the range of from approximately 500.degree. to 600.degree. F. which is too hot to also use to cool the motor and bearings of the pump. Thus, a heat removal arrangement separate from, and which does not employ, the reactor coolant water has been utilized in the prior art. One heat removal arrangement includes an annular hollow jacket surrounding the motor, a set of coils contained in the jacket and surrounding the motor, and other sets of coils located adjacent the upper and lower bearings. The multiple sets of coils are connected in flow communication so as to define a closed path for circulation of an internal coolant fluid therein for cooling the bearings and motor.
The annular jacket of the heat removal arrangement has an inlet and outlet connected in flow communication with an external source of a secondary coolant fluid which can then flow through the jacket over the set of coils contained therein. The secondary coolant fluid is typically at a temperature much lower than the temperature of the internal coolant fluid circulating about the closed path such that the heat carried by the internal coolant fluid gained from cooling the motor and bearings is readily transferred to the secondary coolant fluid through the one set of coils in the jacket.
Use of the above-described heat removal arrangement of the prior art is necessary in reactor cooling systems where the temperature of the reactor coolant water is too high to also be useful in cooling the pump motor and bearings. A drawback of this prior art heat removal arrangement, however, is that it does increase the complexity of the pump.